A
problem of wavelength and size
The Lovell Telescope has a large collecting area and thus provides
high sensitivity but it is not even as good at discerning fine
detail as the human eye! This is because radio wavelengths are
typically 100,000 times longer than those of visible light and
so, to give the same theoretical resolution, a radio telescope
needs to be 100,000 times larger than an optical one. This would
require telescopes many tens of kilometres across, clearly not
a practical possibility. To solve this problem, radio astronomers
use the technique of interferometry in which two telescopes
are linked together electronically to imitate a telescope whose
diameter is equivalent to the distance between them. A simple
interferometer can allow the precise position of a radio source
to be found and it is also possible to learn something about
its structure.

Arrays

The
technique can be extended to link together a number of telescopes
to form an array. At a given instant of time each pair of telescopes
in the array gathers some information about the structure of
the observed radio source. As the Earth rotates, the orientation
and effective baselines of the telescope pairs change allowing
them to gather further information. After 12 or more hours of
observation, sufficient data are acquired to enable a radio
image of the source to be made. The technique is called Earth
Rotation Aperture Synthesis.

Linking
the telescopes

The
signals from the telescopes can be brought together in several
different ways. Where the telescopes are on one site, cables
or waveguides are used. Where greater distances separate the
telescopes, as in MERLIN, microwave radio links are used at
present, but these may be replaced by fibre-optic links for
greater bandwidths and sensitivity. When the telescopes in the
array are spaced across a continent, it is not currently feasible
to combine their signals as they are received. Instead, the
data are stored on large reels of 1" wide magnetic tape
with each block of data precisely "time stamped" to
better than a microsecond. The tapes from each observatory are
then brought to a processing site where they are played back
in precise synchronisation before being combined together in
a computer to form radio images.

What can be achieved?

The
largest array located on one site is the Very Large Array (VLA)
in New Mexico, USA where the telescopes can be up to 36 kilometres
apart. This provides a typical resolution of 1/3 of a second
of arc, similar to that of optical telescopes operating on the
Earth's surface. The 217 km MERLIN array, shown on the inside
cover, can typically provide a resolution of 1/20 of a second
of arc, comparable to images produced by the Hubble Space telescope,
whilst the European VLBI Network (EVN) gives a further factor
of ten improvement.

The
5 km Ryle Telescope operated by Cambridge University where Martin
Ryle and colleagues developed the technique of Earth Rotation
Aperture Synthesis.